1. Field of the Invention
The present invention relates to a method and an apparatus for generating an image data representing an integrated image, and more particularly, to an improvement in integrating respective images of a character, diagram and a picture to generate a halftone dot image data or a binary image data of an integrated image and deliver the same to an output device such as an image scanner.
2. Description of the Background Art
Some current personal computers and work stations are capable of generating an image including characters, diagrams and pictures and transforming the image into a page description language. Such an image is called an "integrated image". Page description language is a language for describing shapes, colors and locations of the characters, diagrams and pictures in accordance with a specific program grammar and an example thereof is a language "Post Script". A program described in a page description language is called "image description program" hereinafter.
FIG. 6 is a block diagram showing an image processing system for generating an image in accordance with an image description program. The image processing system comprises a host processor 10, an image integrating device 20 and an output device 30.
The host processor 10 receives an image description program P.sub.i, a multi-tone image data D.sub.i expressing multi-tone image of a picture and the like through an external interface 13 from an external device (not shown) and decodes the image description program P.sub.i to produce vector data V.sub.l and V.sub.f expressing characters and diagrams, respectively. To the host processor 10, a character font disc 11 for storing character fonts and an image data memory 12 for temporarily storing a multi-tone image data are connected.
The vector data V.sub.l and V.sub.f and the multi-tone image data D.sub.i are transmitted from the host processor 10 to data management unit 21 in the image integrating device 20. The data management unit 21 has a function to control the whole operation of the image integrating device 20.
The vector data V.sub.l and V.sub.f and the multi-tone image data D.sub.i are delivered from the data management unit 21 to the data processing unit 22. The data processing unit 22 performs a coordinate transformation with regard to these data, and additionally, it transforms the vector data V.sub.l and V.sub.f into a raster data R.sub.l and R.sub.f, respectively. The raster data R.sub.l and R.sub.f and the multi-tone image data D.sub.ia obtained in the data processing unit 22 are supplied to a dot generator (a halftone dot image generating unit) 23 and transformed into a halftone image data D.sub.h. The halftone dot image data D.sub.h expresses respective ON/OFF states of halftone dots, in which a single bit is asssigned to each dot cell in each color component. The "dot cell" is respective ones of unit areas forming one halftone dot and the color component is respective ones of R (red), G (green) and B (blue), for example.
The halftone dot data D.sub.h is temporarily stored in an output frame memory 24 in which 1 bit is assigned to each dot cell in each color component, and thereafter, it is outputted to an output device 30 to display or record an integrated halftone dot image.
In the conventional image integrating device 20, the character raster data R.sub.l, the diagram raster data R.sub.f and the multi-tone image data D.sub.ia are separately applied to the dot generator 23, and then a data processing to produce the halftone dot data D.sub.h is performed. For this reason, all of the data R.sub.l, R.sub.f and D.sub.ia are processed under the common condition where resolution is set at the highest resolution in respective data R.sub.l, R.sub.f and D.sub.ia and number of gradation levels is set at the maximum number of gradation levels in respective data R.sub.l, R.sub.f and D.sub.ia. The maximum number of gradation levels is the maximum one of respective numbers of bits expressing optical density levels in the data R.sub.l, R.sub.f and D.sub.ia, for example.
Thus, an efficiency of processing the image data is not necessarily high and there arises the problem that the processing is slow or an efficiency in using an output frame memory is low.
The dot generator 23 generates the halftone dot data D.sub.h by comparing optical density levels of the pixels expressed by the data R.sub.l, R.sub.f and D.sub.ia, respectively, with threshold values or screen pattern data for each pixel in each scanning line. The screen pattern data is previously stored in the dot generator 23. Now then, regions to which characters, diagrams and pictures to be allocated on an imaging plane are often not rectangular. In the prior art, in order to produce a halftone dot data on a region that is not rectangular, it is required that the head coordinates on an image plane in respective main scanning lines in the region is transformed into addresses of the screen pattern data or coordinates on a screen pattern plane, and the screen pattern data is read in accordance with the address. Such a transformation processing is a very complex one, and thus, there arises the problem that a processing speed is lowered in the dot generator 23.